Strange behavior on shrinking a Dynamic Array -- Capacity reduces to 0

[Robert Jacques]

On Wed, 22 Jun 2011 23:38:30 -0400, d coder <dlang.coder at gmail.com> wrote:

> Hello List
>
> As per TDPL and D documentation, shrinking a dynamic array should not
> relocate it. But that is exactly what is happening.
> On shrinking an array, its capacity is getting reduced to 0. Surprisingly
> the capacity after a shrink is even less than the length
> of the array. As a result the array gets relocated as soon as another
> element is added to it.
>
> I have to call reserve again after shrinking it to make sure that the  
> array
> is not relocated.
>
> Is this the expected behavior?

Yes. Capacity is only non-zero for arrays which can safely be extended. If  
you shrink an array from length 10 to length 5, the GC is smart enough to  
know that that there may be a dangling reference to elements [5..10] and  
thus if the length 5 array was appended to, it might stomp on someone  
else's data. Long ago, the GC was dumber and it caused a massive hole in  
the const/immutable system. If you know that no other aliases exist (i.e.  
you're buffering, etc) you can use the function assumeSafeAppend to reset  
the capacity. However, I'd strongly recommend switching to Appender  
whenever you're tempted to use assumeSafeAppend.